Non-aqueous lithium electrochemical cells typically include an anode, a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents and a cathode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the lithium anode. Non-aqueous lithium electrochemical cells are discussed in U.S. Pat. Nos. 4,472,487, 4,668,595, 5,028,500, 5,441,830, 5,460,904, and 5,540,741.
Recently, the lithium metal anode has been replaced with a carbon anode such as coke or graphite intercalated with lithium ions to form Li.sub.x C. In operation of the cell, lithium passes from the carbon through the electrolyte to the cathode where it is taken up just as in a cell with a metallic lithium anode. During recharge, the lithium is transferred back to the anode where it reintercalates into the carbon. Because no metallic lithium is present in the cell, melting of the anode does not occur even under abuse conditions. Also, because lithium is reincorporated into the anode by intercalation rather than by plating, dendritic and spongy lithium growth does not occur.
Various factors influence the performance of electrochemical cells. For instance, electrolyte decomposition will occur with any solvent at high enough potential. In the case of lithium cells, the solvents are organic, aprotic, polar solvents. Conventional solvents are described, for example, in U.S. Pat. Nos. 5,085,952, 4,925,751, 4,908,283, 4,830,939, and 4,792,504. Decomposition of solvents occurs at different rates and at different potentials. In the case of conventional exemplary carbonates, the solvent may be a cyclic carbonate or linear carbonate, yet the same decomposition mechanism applies at different rates. Exemplary organic solvents are .gamma.-butryrolactone, tetrahydrofuran, propylene carbonate, vinylene carbonate, ethylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), butylene carbonate, methyl ethyl carbonate, dipropyl carbonate, dibutyl carbonate, diethoxy ethane, dimethoxyethane, and dioxolane.
Loss of performance due to impurities and undesired side reactions has lead to the selection of solvents and salts which are less reactive with cell components. Unfortunately, this eliminates from use certain solvents and salts which perform better in a cell as compared to their less reactive counterparts. Therefore, what is needed is an understanding of the mechanisms causing undesirable loss of performance and reduce battery life cycle. Although interaction with metallic lithium has now been resolved by eliminating the use of the metallic lithium, yet there still remains the challenge of determining how to prevent undesired side reactions especially those involving formation of gas in cells.